1. Field of the Invention
The present invention relates to an integrated optical channel waveguide system and, more particularly to a waveguide system with a substrate and a waveguide layer structure comprising a first cladding layer, a passive light guiding layer deposited on the first cladding layer, an electro-optical light guiding layer on the passive layer and a second cladding layer.
2. Description of the Related Art
An integrated optical channel waveguide system is known from the article xe2x80x9cFabrication and packaging of integrated chemo-optical sensorsxe2x80x9d published in the Journal xe2x80x9cSensors and Actuators B 1996, vol. 35-36, pages 234-240. In a system described there, optimization of, for example, the sensitivity of the system located in a working or active zone always influences the functioning of the channel system outside the working zone. In this way the optimization is only restrictedly feasible, which negatively influences the sensitivity of the total waveguide system
It is remarked that besides the article cited above similar devices are disclosed in some further documents of which xe2x80x9cIntegrated optic adiabatic devices on siliconxe2x80x9d, published in IEEE Journal of Quantum Electronics, vol. 27 (1991) March no.3, pp. 556-566, discloses a possibility of adiabatically tapering waveguide sections for mode change in order to realise a coupler, a splitter, a multiplexer and a transformer.
The article xe2x80x9cIntegrated optics and new phenomena in optical waveguidesxe2x80x9d, published in Reviews of Modern Physic, vol. 49, no.2. April 1977 pp.; 361-378 discloses a similar device with the aim for a better integration in optical circuitry with a large number of devices.
An article in Journal of Applied Physics, vol. 69 no. 1, June 1991 pp. 7425-7429 discloses an integrated double core layer waveguide structure wherein an upper core layer comprises an adiabatic transition zone in order to examine the difference between adiabatic and non-adiabatic transition of the guided light. This article thus teaches away from optimizing a device through adaption of at least part of the transitions of an integrated waveguide system to reach a sensitive device for sensing or modulation.
Because of the thickness reduction of the active light guiding layer beside the working zone, any influence on the channel system outside the working zone is avoided. As a result of the adapted adiabatic design of the layer-thickness reduction, loss of light power in this zone is also avoided or at least reduced. A simple and reproducible coupling between a light fibre supplying the light and/or a light fibre discharging the light and the optical channel system can also be realized. In the case of the preferred design, the light guiding layer comprises an electro-optic light guiding layer consisting of ZnO and a passive light of Si3N4 and the electro-optic light guiding layer exhibits an adiabatic layer-thickness reduction. Especially the adiabatic layer-thickness transition zone in the electro-optic light guiding layer reduces its layer-thickness outside the modulation working zone to at least almost zero.
Because the light guiding layer here is built up from an active and a passive layer, the layer-thickness reduction outside the active modulation zone can be performed without any restriction because there the passive light guiding layer will be responsible for the light guiding. Every negative influence on the channel system can therefore be avoided without restricting light-guiding throughout the channel system as such.
Conversely the passive light guiding layer can be provided with an adiabatic layer thickness transition zone such that the layer thickness located in the modulation working zone is eventually reduced to zero, and what is more, the space created in this way can be filled up by active light guiding material such as the already mentioned ZnO, by which its sensitivity to the applied voltage there can be further optimized.
In a preferential configuration the layer thickness of an existing Si3N4 passive light guiding layer located in a sensor window working zone, is optimized for sensing using the evanescent field tail of the light to be used. In particular the system is designed as a sensor for measuring chemical and/or physical quantities that influence the refraction-index profile probed by the evanescent field of the light used. Especially at this point in the measurement process, use is made of the evanescent field of the light employed and the sensitivity to the quantity to be measured can be increased by making the sensor windows longer, for example by removing the second cladding layer over a greater length. Consequently, no extra properties of the light used are required for enhancing the sensitivity.
In a further preferential embodiment, the layer thickness of at least one of the cladding layers present in the electro-optical modulation working zone is reduced, in such a way that the active voltage modulation sorts out maximal effects in the active light-guide material, without causing intensity loss of the light used due to an underlying electric conductive substrate material, and/or a locally introduced upper electrode. To obtain a window for the sensing process, the second cladding layer is also locally completely or at least almost removed down to the passive light guiding layer. Due to the fact that in these configurations the layer thickness of one or both of the cladding layers, which preferably consist of SiO2, is reduced at the modulation working zone and similarly is decreased to at least almost zero at the sensor windows working zone, it is possible to optimize the functionality of the waveguide structure, so both optimal sensitivity for evanescent field sensing as well as minimal electric driving voltage modulation can be obtained, without the light confinement, as seen over the whole channel system, being reduced.
In a further preferential design the passive light guiding layer is provided with an adiabatic transition zone leading to a layer thickness, which at that location is optimally adjusted to the mode profile geometry of the light-supplying optic fibre. Especially the layer thickness values of both the passive light guiding layer and the first cladding layer are locally optimized to discriminate between TE and TM polarized light in such a way that the TE is optimally coupled in, and optimally transmitted as well, both with respect to the TM polarized light.
Optimal coupling can be realized with a minimum loss of light, without the structure of the channel set outside the zone needing to be adjusted, by adapting the layer thickness of the passive light guiding layer at the location where the light is coupled in. Discrimination between TE-polarized light and TM-polarized light and efficient TE-acceptation can be realized by a corresponding design of the geometry of the waveguide structure and of a transition zone in the passive waveguide guiding layer respectively, again also without the layer set-up outside that zone being further negatively influenced. Here, discrimination between TE and TM polarized light with the coupling of the light into the interferometer gives the substantial advantage that the whole interferometer measurement process can be carried out with only TE polarized light by which dispersion in the measuring light is reduced, and a significantly more unambiguous interferometer signal is generated.
In a further design a substrate preferentially made of Si is fitted with a V-shaped groove in order to realize a detachable and/or optically adjustable coupling of an optical input fibre and/or output fibre to the light guide channel system. A handy for use transit opening through which simply and with a high degree of precision the input and or output fibre can be inserted and positioned is obtained by attaching a part of the unused V-groove upside down on the V-groove already referred to.
Because of the exact dimensions of the V-groove in the Si, which can be very accurately obtained using etching techniques, the fibre placed there can give an optimal and good reproducible coupling of the light from the fiber into the layer in question of the channel system. As a result of the previously mentioned adiabatic thickness transition of the light guiding layer at this location, the thickness of the channel layer there can be optimally adjusted to the input and/or output fibre. Both properties together result into an optimal coupling with respect to reliability as well as to reduced loss of light. A light source equipped with an optical fibre (pigtailed), such as for example a gas laser or a solid-state (such as a diode laser) can therefore be reliably coupled to the channel system. Thereby a simple detachable coupling can be achieved, having the advantage that in the case of any technical failure, either the laser or the waveguide system can be replaced. On the other hand a permanent coupling can be realized in which case often the light loss or variation in the light loss can be further reduced.
In another preferential configuration, according to the invention, a light guiding system forms part of a Mach-Zehnder type interferometer. Especially such a Mach-Zehnder interferometer, including in both of its branches a sensor window as well as a modulator provided with electrodes, where one of the sensor windows is shielded against the influence of the quantity to be measured.
A Mach-Zehnder interferometer is known from U.S. Pat. No. 5,533,151 or U.S. Pat. No. 5,377,088 and always comprises two arms, where is one arm an external quantity introduces a phase change due to variations of the refractive index. This variation can be extremely accurately measured by comparing this light propagation with the light propagated through the reference arm which cannot be influenced by the measurand.
A practical design for a Mach-Zehnder interferometer according to the invention can be designed. This makes the interferometer less sensitive to external influences other than by the measurand. Especially if such an interferometer is incorporated in an electrical circuit for the derivation of the measurand induced phase shift from the situation at which the anti phase signals have identical intensities. Thus a signal is generated, that is even more unambiguous and more insensitive to external disturbances.
A practical design of an electrical circuit for that purpose is formulated herein. The electrical circuit is particularly equipped with a signal generator for activating a (couple of) modulation electrode(s) in the system, with a threshold-value switch for converting the modulated analogue output signal into a digital signal, and with a digital signal processor for an accurate determination of the value of the measurand from the modulated output signal.